Superconducting ceramic compositions have been discovered only recently. Since 1973, it has been known that certain transition metal alloys showed: superconductivity as high as 23 K (-250.degree. C.) Most of these were based on niobium metal and its alloys. Liquid helium (BP=4 K) was required to maintain the superconductive state. In December 1986, Muller and Bednorz announced the discovery (Georg Bednorz and Alex Muller, Z. Phys. B64, 189 (1986)) of a new ceramic superconducting compound based on lanthanum, barium, and copper oxides. Its critical temperature for superconductivity was close to 35 K. By the following month, the critical temperature Tc for the onset of superconductivity, was raised to nearly 80 K by C. W. Chu and coworkers. (M. K. WU, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, 908 (1987)). This was achieved by changing the composition to yttrium barium copper oxide, approximated by the formula: EQU Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3.
Note that the formula is not exactly stoichiometric. It is believed that this lack of specific stoichiometry contributes most to the onset of superconductivity. Nevertheless, the exact mechanisms connecting superconductivity with chemical composition and stoichiometry remain unknown, even though they are receiving intensive study at this time. The most recent superconducting ceramic compositions announced to date include:
Bismuth Strontium (Calcium ) Copper Oxide: EQU Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y EQU T.sub.c =114 K.
Thallium Barium Calcium Copper Oxide: EQU Tl Ba.sub.2 Ca Cu.sub.2 O.sub.7 EQU Tl Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 EQU Tl Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.11 EQU Tl Ba.sub.2 Ca.sub.4 Cu.sub.5 O.sub.13 EQU T.sub.c =120 K.
Superconducting ceramic compositions are normally prepared by weighing out specific quantities of selected oxides. The combination is thoroughly mixed by conventional means and then fired at elevated temperatures above about 950.degree. C. The induced solid state reaction causes the formation of the desired ceramic composition and structure. Further annealing in an oxygen atmosphere has been shown to improve the superconducting properties of the Y-Ba-Cu-0 compound. The powder so-produced is then processed by conventional means (by compacting) to form a bar which is then used as the superconducting medium.
In other methods, one can prepare superconducting thin films by the use of the electron-beam evaporation method or the vapor phase epitaxial growth method. In the former method, appropriate oxide targets are heated to the point where they evaporate and condense on a suitable substrate. Usually, one sequentially evaporates at least three different oxides, thereby forming the desired stoichiometry and composition. For example, to obtain Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3, one evaporates copper oxide first, then barium oxide and finally yttrium oxide (which melts at 2400.degree. C.). The desired stoichiometry is obtained by controlling the thicknesses of the layers, relative to one another. Actually, the desired stoichiometry of the superconductor (for this composition) is better approximated by: EQU Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.7.0-X
One reason for this lies in the fact that once the three oxides have been evaporated onto a substrate, it is nearly always necessary to anneal the reacted composition in an oxygen-containing atmosphere to attain the superconducting state. As the second layer (barium oxide) is evaporated onto the copper oxide layer, it is still hot enough to react directly with the original layer to form (presumably) Ba.sub.2 Cu.sub.3 O.sub.5. This barium cuprate compound is further reacted with the hot evaporated yttrium oxide to form the final composition. Because the evaporation temperature of Y.sub.2 O.sub.3 exceeds 3400.degree. C., the final product is likely to be deficient in oxygen because of the tendency of such oxides to form lower valence states, i.e.--2CuO--Cu.sub.2 O+Cu (or Y.sub.2 O.sub.3--Y.sub.2 O.sub.3-X), at high temperatures, as is known in the art. This mandates a reannealing step in oxygen atmosphere to restore the critical oxide stoichiometry required for superconductivity. If the mixed oxides are calcined together, the solid state reaction temperature is almost always lower than 1500.degree. C. If the superconductor is to be formed on a lower-melting substrate such as Si (silicon), it has been found to be necessary to evaporate a non-reactive and thermally non-conducting layer such as zirconia (ZrO.sub.2) on the Si surface before one begins to form the superconductor composition.
To control stoichiometry, one controls the thickness of the individual, and sequential, layers evaporated. In one example, a 0.4 micron layer of ZrO.sub.2 was formed as a protective layer. Then a copper oxide layer was deposited; a barium oxide layer was formed, followed by an yttrium oxide layer. This was repeated six times to build up an 18 layer "stack", having a thickness of about 0.75 microns. The stack was then calcined at about 950.degree. C. in oxygen atmosphere, annealed at about 550.degree. C. and then cooled. This process can produce only thin films of a superconducting composition.
In the vapor phase epitaxial growth method, the methodology is similar except that the targets are not vaporized but are sublimed by bombarding the surface with an electrical discharge in an inert ionized gas. For the most part, one may use halide target compositions such as CuCl.sub.2, BaCl.sub.2 and YCl.sub.3 for sublimation sources. Ionized gas molecules such as argon or krypton bombard the target surface and cause metal chloride molecules to be redeposited onto a cooler surface. By proper control of the gaseous discharge conditions, one can cause a single crystal film of oxidic composition to build up in an epitaxial manner. That is, the growth of the superconductor becomes oriented two-dimensionally by the crystal structure of the substrate. For the most part, oxides are not used at all, but are formed during the deposition process because the whole methodology is carried out in an atmosphere containing specified quantities of oxygen gas.
The mechanism of superconductivity in such oxide-based ceramic materials is not at all well understood. The first published suggestion concerning superconductivity was made by Ogg in 1946. He proposed that superconductivity arose in quenched metal-ammonia solutions because of mobile electron pairs. The concept accepted at present is similar and is due to Bardeen, Cooper and Schrieffer (the BCS theory). It suggests that a mobile electron propagating throughout a lattice structure interacts with the bound electrons of the lattice through differences in the electron quantum spin number. If two such electrons form a pair which are bound through opposite spin-pairing (Cooper pairs), then no quantum interaction of the bound pairs can occur with the electrons of the lattice (which still have an electron moment). That the BCS theory has some validity is shown by the following example concerning the structure of the semiconductor--YBa.sub.2 Cu.sub.3 O.sub.6 as compared to that of the superconductor--YBa.sub.2 Cu.sub.3 O.sub.7. Both of these compounds have the idealized cubic perovskite structure. The 1:2:3 perovskite should have nine (9) oxygen atoms in the unit cell whereas the superconductor has less than seven (7). Thus, about one quarter of the oxygen atoms are missing in the superconductor. Yet, the exact mechanism of superconductivity cannot be directly related to this experimental observation.
Another method for preparing superconductors in useful form has been to form the oxides and then compact the so-produced powder into the bar. The bar was then heated on a pedestal by a LASER until it melted, a seed crystal was added, and a fiber was drawn at a controlled rate. The prototype wire was able to carry 30,000 A/cm.sup.2 at 4 K. before it failed. The composition used, Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y, was sintered, then reground and sintered again at least two more times, to achieve a uniform composition. The fiber so-produced was a single crystal but was subject to the shortcomings of all ceramic fibers, namely lack of flexibility and ductility.
All of the above methods given above for producing superconducting compositions are limited in the physical form of the superconductor that they are able to produce. For example, the superconducting powder can be compacted into only rather small pellets or rods. Many devices, especially electromagnetic windings, using superconductors require long wires or ribbons, especially electromagnet windings. As superconductors, especially ceramic superconductors, lack the ductility required for drawing into wires, special manufacturing techniques are required to provide said wires. The limitations on plastic deformation also limits the bending of superconductor wires into the configurations required in the devices.
The superconductivity property is restricted to the individual crystals of the superconductor, and is commonly highly anisotropic in each crystal. Consequently, the orientation of the crystallites is a factor affecting current-carrying capacity. A non-superconducting phase in the current-carrying paths between the crystallites would add resistance that could greatly decrease current-carrying capacity. However, intergranular phases give strength to many ceramics. Superconductors, on the other hand, are likely to suffer losses in superconducting performance from the presence of secondary phases. Many of the common techniques which might be applied to the manufacture of superconducting wires require the presence of binders or other additives which would produce voids or intergranular phases. Some impurities such as water or organic solvents and binders, must be carefully removed to limit formation of voids and shrinkage cracks. Essentially no current is carried through voids or across cracks.
Because of the adverse effects of impurity phases and impurity-caused defects upon superconductor properties, a conceptually ideal technique might use a pure superconductor as starting material and not contaminate it with impurities while producing the final wire configuration of theoretically dense superconductor. I have found that available technologies for the manufacture of metal wires and tubes, and for hot-isostatic-pressing of ceramics to close voids and cracks and to join lengths of ceramics, can be applied to the manufacture of superconducting wires in bent configurations required for devices. These wires are contained in composite structures which are comprised essentially and most simply of the superconductor wire as a core tightly contained in a generally thick-walled metal tube.